NAME

CLUSTER - cluster a table according to an index

SYNOPSIS

CLUSTER [VERBOSE] tablename [ USING indexname ]
CLUSTER [VERBOSE]

DESCRIPTION

CLUSTER instructs PostgreSQL to cluster the table specified by
tablename based on the index specified by indexname. The index must
already have been defined on tablename.
When a table is clustered, it is physically reordered based on the
index information. Clustering is a one-time operation: when the table
is subsequently updated, the changes are not clustered. That is, no
attempt is made to store new or updated rows according to their index
order. (If one wishes, one can periodically recluster by issuing the
command again. Also, setting the table's FILLFACTOR storage parameter
to less than 100% can aid in preserving cluster ordering during
updates, since updated rows are preferentially kept on the same page.)
When a table is clustered, PostgreSQL remembers which index it was
clustered by. The form CLUSTERtablename reclusters the table using the
same index as before.
CLUSTER without any parameter reclusters all the previously-clustered
tables in the current database that the calling user owns, or all such
tables if called by a superuser. This form of CLUSTER cannot be
executed inside a transaction block.
When a table is being clustered, an ACCESS EXCLUSIVE lock is acquired
on it. This prevents any other database operations (both reads and
writes) from operating on the table until the CLUSTER is finished.

PARAMETERS

tablename
The name (possibly schema-qualified) of a table.
indexname
The name of an index.
VERBOSE
Prints a progress report as each table is clustered.

NOTES

In cases where you are accessing single rows randomly within a table,
the actual order of the data in the table is unimportant. However, if
you tend to access some data more than others, and there is an index
that groups them together, you will benefit from using CLUSTER. If you
are requesting a range of indexed values from a table, or a single
indexed value that has multiple rows that match, CLUSTER will help
because once the index identifies the table page for the first row that
matches, all other rows that match are probably already on the same
table page, and so you save disk accesses and speed up the query.
During the cluster operation, a temporary copy of the table is created
that contains the table data in the index order. Temporary copies of
each index on the table are created as well. Therefore, you need free
space on disk at least equal to the sum of the table size and the index
sizes.
Because CLUSTER remembers the clustering information, one can cluster
the tables one wants clustered manually the first time, and setup a
timed event similar to VACUUM so that the tables are periodically
reclustered.
Because the planner records statistics about the ordering of tables, it
is advisable to run ANALYZE [analyze(7)] on the newly clustered table.
Otherwise, the planner might make poor choices of query plans.
There is another way to cluster data. The CLUSTER command reorders the
original table by scanning it using the index you specify. This can be
slow on large tables because the rows are fetched from the table in
index order, and if the table is disordered, the entries are on random
pages, so there is one disk page retrieved for every row moved.
(PostgreSQL has a cache, but the majority of a big table will not fit
in the cache.) The other way to cluster a table is to use:
CREATE TABLE newtable AS
SELECT * FROM table ORDER BY columnlist;
which uses the PostgreSQL sorting code to produce the desired order;
this is usually much faster than an index scan for disordered data.
Then you drop the old table, use ALTERTABLE...RENAME to rename
newtable to the old name, and recreate the table's indexes. The big
disadvantage of this approach is that it does not preserve OIDs,
constraints, foreign key relationships, granted privileges, and other
ancillary properties of the table — all such items must be manually
recreated. Another disadvantage is that this way requires a sort
temporary file about the same size as the table itself, so peak disk
usage is about three times the table size instead of twice the table
size.

EXAMPLES

Cluster the table employees on the basis of its index employees_ind:
CLUSTER employees USING employees_ind;
Cluster the employees table using the same index that was used before:
CLUSTER employees;
Cluster all tables in the database that have previously been clustered:
CLUSTER;

COMPATIBILITY

There is no CLUSTER statement in the SQL standard.
The syntax
CLUSTER indexname ON tablename
is also supported for compatibility with pre-8.3 PostgreSQL versions.